Light emitting diode (LED) arrays are becoming more popular as an image source in both direct view and virtual image displays. One reason for this is the fact that LEDs are capable of generating relatively high amounts of light (high luminance), which means that displays incorporating LED arrays can be used in a greater variety of ambient conditions. For example, reflective liquid crystal displays (LCD's) can only be used in high ambient light conditions because they derive their light from the ambient light, i.e. the ambient light is reflected by the LCDs. Some transflective LCDs are designed to operate in a transmissive mode and incorporate a backlighting arrangement for use when ambient light is insufficient. In addition, transflective displays have a certain visual aspect and some users prefer a bright emissive display. However, these types of displays are generally too large for practical use in very small devices, such as portable electronic devices.
Organic electroluminescent device (OED) arrays are emerging as a potentially viable design choice for use in small products, especially small portable electronic devices, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. OED arrays are capable of generating sufficient light for use in displays under a variety of ambient light conditions (from little or no ambient light to bright ambient light). Further, OEDs can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that OED arrays can be fabricated in a variety of sizes. Also, OEDs have the added advantage that their emissive operation provides a very wide viewing angle.
In spite of all the advantages of OEDs there are still some drawbacks to their use. One of the drawbacks is the fact that light emitted by OEDs is dependent upon the amount of current applied to the OED. Thus, to produce sufficient light for use in displays substantial amounts of current must be applied. As a typical example, with a 64.times.32 array of devices forming a four line by 11 character display the following devices will use approximately the listed amount of power to produce equal light out. If the devices are semiconductor light emitting diodes the display will require approximately 1 Watt (w) of power; organic light emitting diodes will require approximately 150 mw of power; LCDs with a backlight will require approximately 120 mw of power; and reflective LCDs will require approximately 20 mw of power.
One additional problem with the use of LCDs is the contrast ratio, or the difference between light and dark pixels. LCDs have a contrast ratio of 3-6 while OEDs and LEDs have a contrast ratio in excess of 150. Thus, while OEDs and LEDs produce sufficient light for operation under any ambient light conditions, they use a comparatively large amount of power.
An additional problem in the use of OEDs in displays is the generation of the colors necessary to achieve a full color display. Red, green and blue OEDs can be fabricated but they require different organic materials and, thus, each color must be fabricated separately. To overcome this problem, OEDs with a relatively broad spectrum have been used in the prior art and a microcavity is formed on the light outlet. The microcavity can be tuned to one or more frequencies so that a specific color of light is accentuated. See for example, the following publications: "Physics and Device Applications of Optical Microcavities", by H. Yokoyama, Science, Vol. 256, Apr. 3, 1992, pp. 66-70; "Optical Processes in Microcavities", by Yamamoto et al., Physics Today, June 1993, pp. 66-73; "Microcavity Effects in Organic Semiconductors", by Dodabalapur et al., Applied Physics Letters, Vol. 64 (19), May 9, 1994, pp. 2486-2488; Sharply Directed Emission in Organic Electroluminescent Diodes With an Optical-Microcavity Structure", by Tsutsui et al, Applied Physics Letters, vol. 65 (15), Oct. 10, 1994, pp. 1868-1870; Strongly Directed Single Mode Emission From Organic Electroluminescent Diode With a Microcavity", by Tokito et al., Applied Physics Letter, Vol. 68 (19) May 6, 1996, pp. 2633-2635; Efficient Enhancement of Microcavity Organic Light Emitting Diodes", by Jordan et al., Applied Physics Letters, Vol. 69 (14), Sep. 30, 1996, pp. 1997-1999; and "Physics and Applications of Organic Microcavity Light Emitting Diodes", by Dodabalapur et al., Journal of Applied Physics, Vol. 80 (12), Dec. 15, 1996, pp. 6954-6964.
Accordingly, it is highly desirable to produce an OED with enhanced light output which uses less power.
It is a purpose of the present invention to provide a new and improved light emitting device with enhanced light output.
It is another purpose of the present invention to provide a new and improved organic light emitting device with enhanced light output which uses less electrical power.
It is still another purpose of the present invention to provide a new and improved light emitting device with improved colored light output.